1. Technical Field
This application describes method and apparatus for designing and/or implementing a variable flip angle (VFA) MRI (magnetic resonance imaging) spin echo train. For example, a target train of detectable spin-locked NMR (nuclear magnetic resonance) echo signal amplitudes may be defined and a designed sequence of variable amplitude (i.e., variable NMR nutation angle) RF refocusing pulses may be determined for generating that target train of spin echoes in an MRI sequence (e.g., used for acquiring MRI data for a diagnostic imaging scan or the like). Such a designed VFA sequence may be output for study and/or use by an MRI system sequence controller.
2. Related Art
Formation of NMR spin echoes and multiple spin echoes is essential in MRI, such as is done in Fast Spin Echo (FSE) sequences. To generate long series of echoes in MRI, it can be advantageous to sweep the echo train from high amplitude initial RF refocusing pulses to lower amplitude pulses, and sometimes sweep back up to higher amplitude pulses again. The purposes can include (1) generating additional usable echoes and faster scan time, or higher resolution scans in the same time, (2) reduced SAR (specific absorption ratio), or scans less limited by SAR, or (3) favorable motion and artifact characteristics.
When a series of echoes is to be achieved, there are nontrivial design constraints and nontrivial objectives to be met.
The successive echoes usually should have peaks that form some kind of smooth shape or envelope when they are used in forming k-space arrays and then spatial domain images. More specifically, the point spread in resulting images should be kept narrow and relatively free of ghosts or ringing or aliases. The signal level should be maximized, or the signal-to-noise ratio (SNR) should be maximized. The image contrast, such as the amount of T2 weighting or the amount of T1 weighting, should meet any of several possible goals. These contrast objectives may need to be maintained over a range of tissue types. The image may need to have certain characteristics with respect to motion or flow, such as some moving bodily fluids being bright or dark. The image may need to maintain consistent appearance over a range of off-resonance frequencies. Technical characteristics of the RF refocusing pulse train also must be met, such as peak RF power limits, integrated power limits (e.g., based on SAR), or performance over a range of RF imperfections (B1 spatial inhomogeneity.)
For these reasons, it is necessary to have design methods that determine transmit RF refocusing pulse characteristics for each of a series of refocusing pulses. In particular, MR imagers need ways to determine the amplitude of these RF refocusing pulses (i.e., the amplitude typically corresponds to the integrated area of each individual RF refocusing pulse which, in turn, determines the effective NMR nutation angle α of that particular refocusing pulse). The determination of the refocusing pulse amplitudes should result in improved or optimized image quality and image quality objective metrics, subject to the various technical constraints. The resulting output of the MRI scanner pulse sequence design will include an envelope of the train of RF transmit pulse amplitudes, which can be denoted as a “variable flip angle” (VFA), or which also might be called by names such as “variable refocusing angle”, “transmit pulse train envelope”, or the like.
The relevance of such designs is well known, especially for scanning applications like high resolution 3D FSE scanning. The importance of such applications may, in general, increase for scanners with higher main magnetic field strengths.
Design of RF refocusing pulse amplitudes for such echo trains is sometimes done using Bloch equation simulations and sometimes done using Hennig's extended phase graph algorithms. Some relevant prior art teachings are identified below:    1. U.S. Pat. No. 6,456,071—Hennig    2. U.S. Pat. No. 7,164,268—Mugler, III, et al.    3. U.S. Pat. No. 6,850,063—Hennig    4. U.S. Pat. No. 7,227,356—Hariharan, et al.    5. Santyr, et al., “Off-Resonance Spin Locking for MR Imaging,” Magn. Reson. Med., Vol. 32, pages 43-51 (1994)    6. Alsop, “The Sensitivity of Low Flip Angle RARE Imaging,” Magn. Reson. Med., Vol. 37, pages 176-184 (1997)    7. Busse, et al., “Fast Spin Echo Sequences with Very Long Echo Trains: Design of Variable Refocusing Flip Angle Schedules and Generation of Clinical T2 Contrast,” Magn. Reson. Med., Vol. 55, pages 1030-1037 (2006)    8. Busse, et al., “Effects of Refocusing Angle Modulation and View Ordering in 3D Fast Spin Echo,” Magn. Reson. Med., Vol. 60, pages 640-649 (2008)
Some such prior art approaches use T1 and T2 in the simulation and design of such pulse trains. Others use polynomial approximations. In any event, various of these algorithms are used in “variable flip angle” commercial products previously marketed by major MRI system manufacturers.
Often, a target series of echo signal amplitudes is generated. Then, RF refocusing pulses are calculated that result in the desired (target) detectable echo signal levels. If such a design cannot be generated, then the target signal levels are reduced (perhaps by lowering the overall target signal amplitude) and then the calculation of required RF refocusing pulses is repeated until a realizable design can be generated.
A critical building block needed in most such designs is a way to generate a forward simulation where the RF echo signal level of each successive echo can be computed from a series of RF pulse amplitudes and usually additional parameters such as target tissue relaxation values, T1 and T2.
A target performance is specified, such as a detectable echo signal level to be achieved for some range of echoes, or an entire envelope of target echo signal levels with a distinct target signal level for each echo. Such a target may or may not be feasible. It could be infeasible because signal decay (relaxation) of NMR magnetization is such that not enough magnetization signal persists later in an echo train. It could be infeasible because the RF refocusing pulses needed to generate a given echo signal level at a later time exceed an overriding constraint on the transmit RF signal.
The typical design method involves progressively choosing an RF refocusing pulse amplitude to generate each successive echo so that the target echo signal level is achieved. This is done for all successive echoes until a feasible envelope has been fully calculated, or until it is determined that the target train plan is not feasible. Typically, if the target plan is not feasible, an easier target plan with lower echo amplitudes is chosen, and the process is repeated. Also, if the target plan does yield a feasible RF refocusing pulse amplitude train, then it is common to attempt a higher target echo signal level or some other higher level of performance target. Thus, there may be an actual optimization process which attempts to determine or approximate the highest signal level that is feasible, and the RF pulse design that correspondingly generates it.
As noted above, the forward simulation is usually done with either of two widely-used techniques. A first technique simulates the Bloch equations directly, with finite differences generated according to the differential form of the Bloch equations. These Bloch equations are satisfied for each of a series of isochromats, and a summed total signal (over all isochromats) is then determined for each echo. At each time step, NMR magnetization components Mx, My and Mz are calculated based upon the RF pulse, off-resonance, relaxation and the previous values of the three magnetization components.
Alternately, a second technique uses the extended phase graph formalization of Prof. Jurgen Hennig. For each RF refocusing pulse, one determines how much of the signal in each harmonic pathway evolves into each of the neighboring harmonic pathways. With the extended phase graph method, the detected signal at each echo corresponds to the signal in the zeroeth or “unencoded” pathway at the time of the echo. With the extended phase graph calculation, it is possible to use a closed-form calculation at each echo to determine what the RF refocusing pulse amplitude must be to generate the next desired echo signal level.
The extended phase graph algorithm can be limited in effectiveness in that it normally only applies to trains with uniform echo spacing, matched gradient moments per each inter-echo interval, and so forth. If the dephasing across a voxel is not a multiple of 2π, or if the signal characteristics across the voxel are not substantially uniform, then the assumption that the detectable (i.e., transverse Mx) signal comes from the zeroeth transverse harmonic will not hold true, and the calculation will not match the observed signal.
Algorithms that depend upon T1 and T2 in the simulation models also may not fully describe complex tissues. If the actual MR physics is more complex than the assumptions, then the signals will decay at a different rate or develop oscillations, or both.
Either of these forward calculation methods also has a chance of producing progressively increasing error in a VFA RF refocusing pulse amplitude design, as the echo signal train is calculated from echo-to-echo, especially as the signal level becomes nearly unfeasible and the remaining magnetization is nearly exhausted due to relaxation.